Correlations between Geotechnical Parameters and Electrical Resistivity for Limestone at Beni Suef, Egypt

This study presents an integrated geotechnical–geophysical investigation of limestone formations in Ezbet Deir El-Hadid, Egypt. The research combines conventional geotechnical testing with two-dimensional (2D) electrical resistivity tomography (ERT) to develop a cost-effective subsurface characterization methodology. Forty-one boreholes (15–30 m depth) provided direct measurements of unconfined compressive strength (qu) and rock quality designation (RQD), while twenty-five ERT profiles enabled spatial resistivity mapping. Three distinct geological conditions were identified: (1) intact limestone exhibiting resistivity of 21–602 Ω·m, (2) water-saturated zones with reduced resistivity (14–37 Ω·m), and (3) cavities showing higher resistivity (1,038–4,035 Ω·m). An optimized investigation protocol used a Wenner–Schlumberger array with 5 m electrode spacing across multiple 2D profiles, with depth-matched confirmatory boreholes (BH1–BH41). Within a calibrated resistivity range, statistically significant, monotonic relationships between resistivity and rock-mass indicators (RQD and unconfined compressive strength, qu) were established, enabling quantitative cross-validation of geophysical imaging with geotechnical observations. The integrated approach reduced investigation costs by 30–40% while maintaining engineering reliability, offering practical benefits for landslide hazard assessment in carbonate terrains, infrastructure foundation design, and geohazard mitigation. The results delineate the geometry of competent limestone, water-affected or fractured zones, and localized cavities, consistent with borehole stratigraphy, and demonstrate the value of coordinated 2D ERT and borehole programs for robust site characterization and decision support.